Derived type fields do not survive (typeless) serialization/deserialization. #2189

bwloper · 2025-04-22T06:13:46Z

bwloper
Apr 22, 2025

Hi. I am experimenting with MessagePack Typeless serialization. I have noticed that if I have a field of type base_type and I assign it an object of type derived_type derived from base_type, the object appears to come back as a base_type type object after deserialization. Is there a way to have the serializer retain the runtime type of the object if it is different from the field type in cases like this? This is also true of elements in a typed collection.

`

public class DerivedItemType : ItemType
{
    public int TestField { get; set; } = 4;
}

public class ItemType
{
    public int Id { get; set; }
}

public class ClassToSerialize
{
    public ItemType item = new DerivedItemType() { Id = 1 };

    public List<ItemType> list = new List<ItemType> { new DerivedItemType { Id = 2 }, new DerivedItemType { Id = 3 } };
}

    public static void Main()
    {
        ClassToSerialize obj = new ClassToSerialize();
        byte[] bytes = MessagePackSerializer.Typeless.Serialize(obj);
        ClassToSerialize clone = MessagePackSerializer.Typeless.Deserialize(bytes) as ClassToSerialize;
    }

`

In the above code, the "clone" object has only fields and list elements of type ItemType, whereas the original object has objects of type DerivedItemType. Therefore data from the derived type (TestField in this case) is lost. Thanks!

AArnott · 2025-05-28T00:05:22Z

AArnott
May 28, 2025
Maintainer

Look into unions and typeless in the readme.

3 replies

AndreyPolovodov Sep 23, 2025

Hi, @AArnott!
I have the same question. If I use List<object> instead of List<BaseType>, everything works fine and I get the real types in the list. But it feels a bit ugly to use List<object>. I also don’t want to use the union feature, because my task is to migrate a fairly large codebase from BinaryFormatter to MessagePackSerializer.Typeless without doing a lot of refactoring. BinaryFormatter handled this situation as expected.

So, what’s the simplest way to get the same behavior as BinaryFormatter? Maybe some custom formatter, resolver… I’m not sure where to start digging. What would you recommend?

    [Test]
    public void ListOk()
    {
        var t = new DerivedClass();
        var tBase = new BaseClass();
        var list = new List<object> { t, tBase };
        var ms = new MemoryStream();
        MessagePackSerializer.Typeless.Serialize(ms, list);
        ms.Position = 0;
        var desT = MessagePackSerializer.Typeless.Deserialize(ms);
        Assert.IsInstanceOf<List<object>>(desT);
        var asList = (List<object>)desT;
        Assert.IsInstanceOf<DerivedClass>(asList[0]);
    }
    
    [Test]
    public void ListFail()
    {
        var t = new DerivedClass();
        var tBase = new BaseClass();
        var list = new List<BaseClass> { t, tBase };
        var ms = new MemoryStream();
        MessagePackSerializer.Typeless.Serialize(ms, list);
        ms.Position = 0;
        var desT = MessagePackSerializer.Typeless.Deserialize(ms);
        Assert.IsInstanceOf<List<BaseClass>>(desT);
        var asList = (List<BaseClass>)desT;
        Assert.IsInstanceOf<DerivedClass>(asList[0]);
    }

    [Test]
    public void BinaryFormatterOk()
    {
        var t = new DerivedClass();
        var tBase = new BaseClass();
        var list = new List<BaseClass> { t, tBase };
        var ms = new MemoryStream();
        var bf = new BinaryFormatter();
#pragma warning disable SYSLIB0011
        bf.Serialize(ms, list);
        ms.Position = 0;
        var desT = bf.Deserialize(ms);
#pragma warning restore SYSLIB0011
        Assert.IsInstanceOf<List<BaseClass>>(desT);
        var asList = (List<BaseClass>)desT;
        Assert.IsInstanceOf<DerivedClass>(asList[0]);
    }

AArnott Sep 23, 2025
Maintainer

You can apply [MessagePackFormatter(typeof(TypelessListFormatter<BaseClass>))] on the member and implement TypelessListFormatter<T> to use the TypelessFormatter for each element.

AndreyPolovodov Sep 25, 2025

That helps, thank you

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Uh oh!

Derived type fields do not survive (typeless) serialization/deserialization. #2189

Uh oh!

{{title}}

Uh oh!

Uh oh!

{{editor}}'s edit

{{editor}}'s edit

Uh oh!

Replies: 1 comment 3 replies

Uh oh!

{{title}}

Uh oh!

Uh oh!

{{title}}

Uh oh!

Uh oh!

{{editor}}'s edit

{{editor}}'s edit

Uh oh!

Uh oh!

{{title}}

Uh oh!

Uh oh!

{{title}}

Uh oh!

Select a reply

Uh oh!

Uh oh!

Derived type fields do not survive (typeless) serialization/deserialization. #2189

Uh oh!

Uh oh!

bwloper Apr 22, 2025

Replies: 1 comment · 3 replies

Uh oh!

AArnott May 28, 2025 Maintainer

Uh oh!

Uh oh!

AndreyPolovodov Sep 23, 2025

Uh oh!

AArnott Sep 23, 2025 Maintainer

Uh oh!

AndreyPolovodov Sep 25, 2025

bwloper
Apr 22, 2025

Replies: 1 comment 3 replies

AArnott
May 28, 2025
Maintainer

AArnott Sep 23, 2025
Maintainer